The protocluser is 12.4 billion light-years away and its light started traveling to us when the universe was only 1.4 billion years old.
Calculations suggest that by the present day, hundreds more galaxies would have been eaten up by the cluster.
It could have a mass equivalent to 1,000 trillion suns, which would make a galaxy that is the largest known object in the universe.
ALMA, one of the telescopes in Chile, was used to spot and predict the movements of 14 galaxies that merged into one. The images were spotted more than 90 per cent across the universe, 12.4 billion years ago
Astronomers believe there are at least 14 galaxies packed into an area only four times the diameter of the Milky Way’s galactic disk.
Its individual galaxies are forming stars as much as 1,000 times faster than our home galaxy.
The resulting galaxy cluster will rival some of the most massive clusters we see in the universe today.
This extreme level of congestion and star birth rate is unprecedented.
'[It] seems like we're catching a cluster in the process of being assembled,' says study co-author Chris Hayward, an associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute in New York City.
'This is the missing link in our understanding of how clusters form.'
Using mathematical models based on real observations, an international team of scientists has been able to predict the cluster's movement.
However, how the assembly of galaxies became so big so fast 'is a bit of a mystery,' says study co-author Scott Chapman, Professor in astrophysics at Dalhousie University.
In total, the protocluster contains around 10 trillion suns' worth of mass and formed around 1.4 billion years after the Big Bang, far earlier than it was previously thought to happen
'It wasn't built up gradually over billions of years, as you might expect,' explains Tim Miller, a PhD candidate at Yale University and lead author of one of the papers.
'This discovery provides an incredible opportunity to study how galaxy clusters and their massive galaxies came together in these extreme environments.'
Current theoretical and computer models suggest that protoclusters as massive as this one should have taken much longer to evolve.
Astronomers previously thought that these events occurred around three billion years after the Big Bang.
When they realised they were occurring at half that age, scientists rushed to find a new explanation.